The interlacing process of the prior art broadcast systems, such as the 525 line-per-frame, 30 frame-per-second (525/30) NTSC system or the 625/25 PAL system, induces adverse effects. For example, line structure is visible because the interlaced process divides the 525-line picture or frame into two successive 262.5-line fields, reducing the vertical resolution. Another adverse effect is the interline flicker, resulting from the corresponding picture elements of the two successive fields being displaced in both time and space.
In the past, these adverse effects were not considered to be serious because of the low pass filter effect resulting from the interaction of the monitor with the viewer's eyes, and the viewer's subconscious tendency to ignore them. However, the use of wide-screen displays and the increased visual quality requirements of improved definition television systems (IDTV) and high definition television systems (HDTV) necessitate enhanced picture quality. One approach to achieve this, which is known in the art, is the progressive scan technique. In one form of progressive scan system, the horizontal scan rate is multiplied, i.e., doubled, and each line of video is displayed twice, providing a displayed image having reduced line visibility. In this scan system, the added video signal lines duplicate the original scan lines as described in U.S. Pat. No. 4,415,931 to Dischert entitled TELEVISION DISPLAY WITH DOUBLED HORIZONTAL LINES.
In another form of a progressive scan system the "extra" lines for the display are obtained by interpolation between adjacent horizontal lines of the incoming video signal, as described in U.S. Pat. No. 4,400,719 to Powers entitled TELEVISION DISPLAY SYSTEM WITH REDUCED LINE SCAN ARTIFACTS.
It is also known in the art that an "adaptive" progressive scan system can be applied which automatically switches between an intrafield interpolation filter and an interfield interpolation filter, improving the displayed image quality for the motion pictures and the still pictures, respectively. This method is described in U.S. Pat. No. 4,598,309 to Casey, entitled TELEVISION RECEIVER THAT INCLUDES A FRAME STORE USING NON-INTERLACED SCANNING FORMAT WITH MOTION COMPENSATION.
When implementing a progressively scanned receiver with digital processing circuits, random access memory (RAM) is typically utilized to store incoming sampled and digitized video signals. For NTSC standard video signals, the sampling clock frequency is often selected to be four times that of the color subcarrier frequency (f.sub.sc) or about 14.3 MHz, with a period of about 70 nanoseconds. Then, in the progressive scan processor, the read-out frequency of the random access memory will be as high as 28.6 MHz (8f.sub.sc), because the displayed data rate is twice that of the incoming data rate, as described in the prior art. This sampling clock frequency and the standard NTSC line period of about 63.5 microseconds requires 910 RAM memory locations to store the video samples or "pixels" (picture elements). The adaptive interpolation system requires about 1.9 M bit of field memory, which is too large to be integrated into an ASIC chip with other necessary functional blocks.
The progressive scan processor increases vertical resolution while reducing adverse effects such as visible line structure and line crawling. However, such a high rate of input data into the progressive scan processor (PSP) of the prior art required a memory that is too large and expensive to be practical. Even with external memory an ASIC PSP chip remains difficult to design by a low-cost process because the required operational frequency is high.
A need therefore exists to reduce the signal data rate into a progressive scan processor to lower both the memory requirement and the high operational clock frequency. However, this process must reduce the adverse effects encountered in an interlace-scanned video receiver.
It is an object of the invention to provide an improved television display without the adverse effects due to the interlaced scanning process.
It is another object of the invention to increase the resolution of a television display by a progressive scanning with a higher line-rate.
It is a further object of the invention to improve the progressive scanning process by reducing the data rate into the progressive scan processor to reduce both the required memory and the operational frequency.
In accordance with the present invention, the standard line-rate incoming interlaced video signals are first sub-sampled. The signals in the first of two successive fields are sub-sampled with diagonal offset relative to the signals in the second field. Then, interpolated signals are generated from the sub-sampled interlaced signals by either a line memory or a field memory. Finally, a higher displayed line rate is utilized to interpolate each generated line signal using a diagonal offset between two adjacent, sub-sampled interlaced line signals. Also, the bandwidth of the incoming interlaced video signals can be limited before sub-sampling them, preventing the introduction of error in high frequency analysis known as the aliasing effect.